168 DISCUSSION OF THE DESICCATION QUESTION. 



lias nothing to do with the absence of the forests. Examination, however, 

 shows that the soil in such regions is of an exceedingly fine texture ; so 

 much so, that it polishes the tools with which it is cultivated, instead of 

 scratching them. Careful investigation also reveals the fact that all through 

 these prairie regions the occasional presence of clumps or belts of trees is 

 invariably associated with the existence, in such localities, of a coarser vari- 

 ety of soil. For instance, a grove — as such isolated patches of forest are 

 usually called — in which the trees are thickly crowded together and flour- 

 ishing will be seen in the midst of an area of perhaps hundreds of square 

 miles in extent over which not a single tree is growing. Examination of 

 such a locality will sliow at once that the grove covers a patch of gravelly 

 soil, while the surrounding treeless area, which is usually lower and flatter 

 than the spot occupied by the trees, is covered with the fine prairie soil, the 

 character of which is so well known at the West, and which lias been repeat- 

 edly described in the different State geological reports. 



In regard to the effect of this peculiar fineness of the soil in preventing 

 the growth of forests in the " prairie region " of the Mississipj^i Valley, the 

 writer speaks from careful and long-continued examination of that portion 

 of the country. That the same conditions hold good in other regions, 

 and especially over a large ai-ea of the treeless part of the South American 

 continent, seems, also, hardly to be doubted. There are extensive areas in 

 that country where the precipitation is certainly more than abundant, and 

 where the temperature conditions are perfectly favorable, yet where grasses 

 and various flowering plants and shrubs flourish, to the exclusion of trees. 

 Here the fineness of the soil seems to be the essential cause of the j^eculiar 

 character of the vegetation, as it most certainly is in the prairie region of 

 North America. 



The j^eculiar influence of texture of soil in favoring the growth of the 

 grasses in preference to arboreal vegetation, although advocated many years 

 ago by the present writer.* has been overlooked by most of the investigators 

 into problems of this character, and another theorj' has been maintained, 

 especially by the distinguished German physical geographer Peschel. This 

 theory is to the effect that it is not the insufficiency of moisture, in such 

 treeless regions, Avhich prevents the growth of the forests, but its unequal 

 distribution through the year. This theory was applied by Peschel particu- 



* 111 tlie Geology of Iowa, 1858, Vol. I. p. 24. See, also, The American Naturalist for Oetober and November, 

 1876. 



